SAW technology has found a number of applications in the electronics and RF art. Due to the fact that SAW wavelengths are typically 10.sup.5 times shorter than that of electromagnetic waves having a corresponding frequency, SAW technology has found particular applications where miniaturisation is important or desirable. One such application is the use of SAW filters in radio telephones where the typically small size and weight of SAW filters is highly advantageous over conventional technologies, such as ceramic filters, dielectric filters, and filters using magnetostatic principles. Generally, it is a requirement of such filters that they have low-loss, typically insertion losses of 1.about.3 dB for RF use, although for IF filters an insertion loss of 5.about.13 dB is acceptable.
A typical example of a conventional SAW filter is a SAW filter in which SAW energy is transferred between two spaced apart interdigital transducers (IDTs). The IDTs comprise two sets of equally spaced metal strips (electrode fingers), which are formed on the surface of a piezoelectric substrate. The electrode fingers in each set are typically electrically coupled together by bus-bars and are interleaved (interdigitated) with the electrode fingers of the other set. This arrangement can generate SAWs in both directions transverse to each electrode finger when an electrical voltage is applied between the sets of electrode fingers, and it can also generate an electrical voltage when SAWs are incident on the electrode fingers. These processes are most efficient when the frequency of the SAWs is such that the periodicity of the electrode fingers in each set is close to or the same as the SAW wavelength, or some multiple of this frequency. In the simplest form of IDT, the spacing between adjacent electrode fingers of a set of electrode fingers is one SAW centre wavelength, i.e. one electrode finger per period. However, it is possible to have more than one finger per SAW wavelength (period).
A particular path for a surface acoustic wave comprising SAW elements such as IDTs and/or reflection gratings is known as a track.
For conventional filters, to a first order the lengths of the transducers substantially determine the bandwidth of the filter since BW .alpha..sup.1 /.sub.L where L is the length of the transducers. Thus, the more narrow the bandwidth requirement the longer the transducers need to be. Increasing the length of transducers to decrease the bandwidth of the filter mitigates against advantages of small size usually associated with SAW devices. In particular, there has recently been a requirement for narrow-band filters for portable apparatus, and it is desirable to be able to provide such narrow-band devices without any increase in the size of the components and preferably with a decrease in the size of the components relative to conventional components and known SAW devices.